Ferritic heat-resistant steel

ABSTRACT

Disclosed is a ferritic heat-resistant steel which has the following chemical composition (by weight): C: from 0.01% to less than 0.08%; Si: 0.30-1.0%; P: 0.02 or less; S: 0.010% or less; Mn: 0.2-1.2%; Ni: 0.3% or less; Cr: 8.0-11.0%; Mo: 0.1-1.2%; W: 1.0-2.5%; V: 0.10-0.30%; Nb: 0.02-0.12%; Co: 0.01-4.0%; N: 0.01-0.08%; B: not less than 0.001% and less than 0.010%; Cu: 0.3% or less; and Al: 0.010% or less, provided that the chemical composition satisfies the following equations: Mo (%)+0.5×W (%)=1.0-1.6, and C (%)+N (%)=0.02-0.15%, and which comprises a tempered martensite single-phase tissue produced by thermal refining and contains 30% by weight or less of δ ferrite.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 12/278,306, which is a National Stage of InternationalApplication No. PCT/JP2007/051968, filed Feb. 6, 2007, which claimspriority under 35 U.S.C. §119 of Japanese Patent Application2006-029009, filed Feb. 6, 2006. The entire disclosure of the parentapplication is expressly incorporated by reference herein.

TECHNICAL FIELD

The present invention relates to a ferritic heat-resistant steel, andmore specifically, to a high-strength steel for boiler steel pipessuitable for an ultra supercritical pressure thermal power plant withimproved power generation efficiency.

BACKGROUND ART

Recently, in thermal power plants, the temperature and pressure of steamconditions have been raised for improvement in plant efficiency in viewof global environmental issues such as CO₂ emissions reduction. Now,plants which can raise the steam temperature from the current maximummain steam temperature around 600° C. to 650° C., ultimately, to 700° C.have been developed and studied domestically and internationally.According to such a steam temperature rise, a heat-resistant steel withhigh creep rupture strength is required at high-temperature and pressureresistant portions of a boiler. Therefore, for heat transfer pipes of aboiler, an austenitic heat-resistant steel having excellent corrosionresistance and creep rupture strength has been increasingly used.

On the other hand, in the case of a thick pipe with a large diameterlike a header or piping, when this austenitic heat-resistant steel isused, the linear expansion coefficient becomes higher and the heattransfer coefficient becomes smaller than those of the ferriticheat-resistant steel. Therefore, when the plant starts or stops, a greatthermal stress is applied to these header and piping and the header andpiping are easily damaged due to thermal fatigue. In addition, theincrease in material cost and process cost causes an economic problem.Therefore, development of new ferritic heat-resistant steel with highcreep rupture strength and excellent corrosion resistance has beendemanded. As an example of such ferritic heat-resistant steel, amaterial obtained by increasing the proportion of Cr in a conventionalsteel of 9% Cr and 1% MoNbV and adding alloy elements of W and Co, etc.,has been proposed (for example, Japanese Patent No. 2528767).

However, for example, when a ferritic heat-resistant steel is used for aboiler that will reach a steam temperature around 650° C., the ferriticheat-resistant steel contains many W and other alloy elements, so thatduring long-term use, aggregation and coarsening of fragileintermetallic compounds or carbide occur. Therefore, it has been foundthat the creep rupture strength lowers during long-term use for severaltens of thousands of hours or longer. Particularly, at a hightemperature around 650° C. much higher than 600° C., so-called flutingthat involves sudden lowering in creep strength after about several tensof thousands of hours is a great obstacle in the development of ahigh-Cr steel (for example, Non-Patent Document 1).

These header and piping are exposed for a long time to high-temperaturesteam that is an internal fluid of a boiler steel pipe. When the high-Crferrite steel is used at a high temperature around 650° C., productionof oxidized scale due to steam becomes pronounced (for example,Proceedings of Workshop, pp. 153 described above), so that the scalegrowth and exfoliation, and scale scattering to the downstream also comeinto question. To solve these problems, a special method by addition ofnoble metals (Non-Patent Document 2) has also been proposed, however,this greatly increases the material cost, so that it has not been madepracticable.

According to Non-Patent Document 3, improvement in creep rupturestrength by adding components of V, Nb, N, Mo, W, and B, etc., tohigh-chromium steel containing 9 to 12% of chromium is described. Someof the high-chromium steels of the Non-Patent Document 3 realize creeprupture strength at a temperature around 600° C. during long-term use,however, the creep rupture strength at a temperature around 650° C. ismuch lower than that at 600° C.

The inventors previously developed a ferritic heat-resistant steel withhigh creep rupture strength at a temperature around 650° C. and appliedfor a patent (Japanese Laid-Open Patent Publication No. 2005-23378).

-   Patent Document 1: Japanese Patent Publication No. 2528767-   Patent Document 2: Japanese Laid-Open Patent Publication No.    2005-23378-   Non-Patent Document 1: R. Viswanathan et al., “Materials for    Ultrasupercritical Coal-fired Power Plant Boilers” (p. 146-157); R.    Blum et al., “Materials Development in Thermie Project for 700° C.    USC Plant” (pp. 158-176), Committee of the 8th Ultra-Steel Workshop,    Steel Research Center, National Institute for Materials Science,    Jul. 22, 2004-   Non-Patent Document 2: Haruyama et al., “Influence of Pd addition on    the steam oxidation behavior of 9Cr ferrite steel,” Material and    Process, The Iron and Steel Institute of Japan, March 2003, Vol. 16,    No. 3, p. 648-   Non-Patent Document 3: Gabrel J et al., “Status of development of    the VM12 steel for tubular applications in advanced power plants,”    Proceedings of the 8th Liege Conference Part II, Materials for    Advanced Power Engineering 2006, Forschungszentrum Jülich GmbH,    September 2006, p. 1065-1075

DISCLOSURE OF THE INVENTION Problem to be Solved by the Invention

The ferritic heat-resistant steel described in Patent Document 2,developed earlier by the present inventors, has high creep rupturestrength at a temperature around 650° C., however, its long-termstrength is insufficient, and there is room for improvement in steamoxidation resistance (improvement in property of oxidation due to steaminside the pipe). Thus, the conventionally proposed alloys areinsufficient in property as a material to be used at a temperaturearound 650° C. Further, a ferritic heat-resistant steel which has highcreep rupture strength at a temperature around 650° C., and maintainsstably the strength for a long period of time, and is excellent in steamoxidation resistance has been demanded.

An object of the present invention is to provide a high-strengthferritic heat-resistant steel which is excellent in long-term creeprupture strength and steam oxidation in the case where it is used at atemperature around 650° C. in comparison to conventional materials.

Means for Solving the Problem

The object of the present invention is achieved by the followingsolution means.

A first aspect of the invention provides a ferritic heat-resistant steelwhich has the following chemical composition (by weight): carbon (C):0.01-0.10%; silicon (Si): 0.30-1.0%; phosphorus (P): 0.020% or less;sulfur (S): 0.010% or less, manganese (Mn): 0.2-1.2%; nickel (Ni): 0.3%or less; chromium (Cr): 8.0-11.0%, molybdenum (Mo): 0.1-1.2%; tungsten(W): 1.0-2.5%; vanadium (V): 0.10-0.30%; niobium (Nb): 0.02-0.12%;cobalt (Co): 0.01-4.0%; nitrogen (N): 0.01-0.08%; boron (B): not lessthan 0.001% and not more than 0.010%; copper (Cu): 0.3% or less;aluminum (Al): 0.010% or less, provided that the amount of (Mo %+0.5×W%) is limited to 1.0-1.6, and the amount of (C %+N %) is limited to0.02-0.15%, and which comprises a tempered martensite single-phasetissue produced by thermal refining.

A second aspect of the invention provides the ferritic heat-resistantsteel according to the first aspect, which has the chemical compositionin which the amount of (Al %+0.1×Ni %) is limited to 0.02% by weight orless.

A third aspect of the invention provides the ferritic heat-resistantsteel according to the first or second aspect, containing 30% by weightor less of δ ferrite.

Carbon (C) is an element important for forming carbide (M23C6, M6C,M7C3, etc.) that contributes to strengthening of a high-Cr ferriticheat-resistant steel. Conventional practical steels require about0.1-0.12% of carbon, however, if the carbon content exceeds 0.10%, itpromotes aggregation and coarsening and lowers the creep rupturestrength, so that in the present invention, the carbon content is set to0.10% or less to stabilize long-term creep strength. The lower the Ccontent, the higher the creep rupture strength, however, if the Ccontent is less than 0.01%, the toughness deteriorates, so that the Ccontent is set to 0.01-0.10% as a practical steel.

Fine control of the C content requires advanced techniques insteelmaking, however, in particular, by reducing the C content fromabout 0.1% of a conventional steel to 0.08% or less, the Ac1 point(transformation point) is greatly raised, and the long-term creeprupture strength can be further improved.

Silicon (Si) was an element necessary as a deoxidizing agent forproducing steel. However, recently, vacuum deoxidation has becomepossible, and it becomes possible to obtain a low-Si steel by vacuumdeoxidation, and vacuum deoxidation has also been used for obtaining ahigh-Cr heat-resistant steel. Si is an element that improves theoxidation resistance, and to obtain steam oxidation resistance necessaryas a 600° C.-class boiler material, at least 0.30% of Si is necessary.To obtain steam oxidation resistance sufficient as a 650° C.-classboiler material, it is generally preferable that the scale thickness is200 micrometers or less.

On the other hand, if a large amount of Si over 1.0% is added,production of Laves phase of tungsten (W), etc. are promoted, andductibility is lowered due to grain boundary segregation, etc.Therefore, when attaching importance to the creep strength, the Sicontent tends to be made low, and this is an obstacle in using the steelat a temperature around 650° C.

However, in the present invention, it was found that according to theeffect of reducing M23C6 carbide aggregation and coarsening by aluminum(Al) described later, high creep strength could be obtained even if theSi content was increased. Therefore, to obtain steam oxidationresistance sufficient as a 650° C.-class boiler material, the Si contentis set to 0.30-1.0%. When emphasizing the ductibility of steel, the Sicontent is set to 0.30-0.80% (FIG. 1) since a high Si content lowers theductibility.

0.2% or more of manganese (Mn) is necessary as a deoxidizing agent, andat the same time, manganese is useful as an austenite forming elementfor restraining production of δ ferrite. However, if it is added by anamount over 1.2%, the Ac1 transformation point lowers, and the creepstrength lowers. Therefore, the Mn content is limited to 0.2-1.2%.

Phosphorus (P) and sulfur (S) are low-melting point elements, so that iftheir contents are high, they harmfully influence the creep rupturestrength, so that the lower the contents of these the better. However,it is difficult to completely remove P and S, and if their contents areextremely low, it causes an increase in material cost, so that it is notnecessary to make the contents extremely low, and P is limited to 0.020%or less and S is limited to 0.010% or less.

Chromium (Cr) is an important element that provides a steel withoxidation resistance and steam oxidation resistance, however, if itscontent is more than 11%, it forms δ ferrite and lowers the toughness,and precipitation of M23C6 carbide, etc., and growth coarsening due tothe precipitation become pronounced and lower the long-term creepstrength. Therefore, the Cr content must be set to 11% or less. In thepresent invention, the steam oxidation resistance is improved by addinga large amount of Si, however, its effect is insufficient if the Crcontent is less than 8%, so that the Cr content is set to 8 to 11%.

Molybdenum (Mo) is an effective element for increasing the creep rupturestrength by fine precipitation of carbide. Therefore, the molybdenumcontent necessary for strengthening. by precipitation of carbide is atleast 0.1% or more, however, if molybdenum over 1.2% is added and 0.1%or more of W is contained, δ ferrite is produced, and aggregation andcoarsening of M23C6 carbide containing Mo occur and lead to creepstrength lowering. Therefore, the Mo content is set to 0.1 to 1.2%.

Tungsten (W) is an element most important for increasing the creeprupture strength of this steel by precipitation strengthening of carbideand solution hardening in base metal. It had been conventionally saidthat addition of about 3% of W is effective, and if 4% or more is added,M23C6 carbide and Laves phase (Fe2W) including W aggregate and coarsenand lower the creep rupture strength. However, it was found that even anaddition of about 3% lowered the long-term creep rupture strength, sothat the content is reduced to be low and is set to 2.5% or less withwhich creep rupture strength of 100 N/mm² or more at 650° C. after20,000 hours is obtained (FIG. 2). If the content is less than 1.0%,from the data shown in FIG. 2, it is presumed that the creep rupturestrength cannot be improved, so that the content is set to 1.0 to 2.5%.W influences the creep strength in combination with Mo, so that thecontent of W only is regulated, and the value of (Mo %+0.5×W %) is setto 1.0 to 1.6% effective for improvement in creep rupture strength.

Cobalt (Co) is an austenite forming element, and prevents production ofδ ferrite without greatly lowering the Ac1 transformation point, so thatit is an important element. The amount of δ ferrite changes depending onquantities of other alloy elements to be added, however, by adding atleast 0.01%, and 4.0% at most of cobalt (Co), production of δ ferritecan be sufficiently prevented.

Vanadium (V) is an element that forms carbide of V and effectivelyincreases the creep rupture strength at a relatively small amount, andat least 0.10% must be added for forming carbide of V. However, if thecontent is more than 0.30%, the carbide of V aggregates and coarsens andlowers the creep rupture strength, so that the content is set to 0.10 to0.30%.

Niobium (Nb) forms Nb (C, N) (carbonitride of niobium) that is stablecarbonitride and can increase the creep rupture strength even withsupply of a small amount, however, if the content is more than 0.12%, itis not good for long-term strength although short-term strength isimproved. If it is less than 0.02%, precipitated Nb (C, N) is in shortsupply and strengthening is insufficient, so that the content is set to0.02 to 0.12%.

Nitrogen (N) increases the creep rupture strength by precipitationstrengthening by forming nitride of V and solution hardening of itself.However, nitrogen over 0.08% forms too much nitride and causesaggregation and coarsening, and lowers the creep rupture strength, sothat the nitrogen content is set to 0.08% or less. If the nitrogencontent is less than 0.01%, the effect of increasing the creep rupturestrength is insufficient, so that the nitrogen content is set to 0.01 to0.08%. The creep rupture strength based on N has a close correlationwith the amount of C, and when (C %+N %) is limited to 0.02 to 0.15%,the highest creep rupture strength is obtained.

Nickel (Ni) is an element effective for improving the toughness andrestraining production of δ ferrite, and in a conventional boiler steel,it is added by about 0.5% without special limitation. However, it wasfound that addition of nickel greatly lowered the Ac1 transformationpoint and harmfully influenced the long-term creep strength, so that inview of the creep rupture strength, it is preferable that the nickelcontent is lowered to be not more than 0.1%. However, for this, at thetime of steelmaking, the amount of nickel to be mixed from scrap steel,the furnace wall, and ladle, etc., must be minimized, and this increasesthe steelmaking technical limitations, so that the upper limit of thenickel content is set to 0.3% as a practical steel. Ni may not becontained.

Aluminum (Al) is conventionally added as a deoxidizing agent and acrystal grain refining agent. However, excessive addition of 0.010% ormore of Al captures nitrogen that is effective for improving the creepstrength as Al nitride, and is incrassated on the surface of M23C6carbide and promotes diffusion of Cr, and accelerates aggregation andcoarsening of M23C6 carbide. The present inventors also found that whenthe Al content exceeded a predetermined value, even if it was a verysmall amount, it greatly lowered the long-term creep strength afterseveral tens of thousands of hours at a temperature around 650° C. In aconventional practical steel, the toughness is improved by adding Al upto about 0.03%, however, in the heat-resistant steel of the presentinvention, by reducing the Al content to 0.01% or less, the long-termcreep strength at 650° C. is greatly increased.

It was conventionally very difficult to reduce the Al content to such anextremely low level. However, recently, it became possible tomanufacture an extremely-low level Al steel by means of vacuum carbondeoxidation method. The increase in Si content in the steel of thepresent invention is also one of the features, and even if thedeoxidation effect of Al is lost, the deoxidation effect of Si can beused, so that the amount of oxygen in the steel can be reduced. Al maynot be contained.

FIG. 3 shows the creep rupture strength at 650° C. in materials withvariously changed Al quantities and Ni quantities on the vertical axis,and the Al quantities on the horizontal axis. As creep rupture strengthat 650° C. after 100,000 hours, to obtain about 100 N/mm², strength over100 N/mm² after 20,000 hours is required. Therefore, for example, whenthe Ni amount is about 0.3%, the Al amount is reduced to 0.005% or less.When the Ni amount is 0.1% or less, the allowable range of the Al amountcan be widened.

The steel of the present invention has a feature in that Al and Ni thatare elements especially harmful for stabilizing the creep strength arereduced, and reduction of these is essential. The same test results asin FIG. 3 are shown in FIG. 4 by rewriting the Al amount on thehorizontal axis into Al amount+(0.1×Ni amount). From these test results,as a range of the Al amount and Ni amount with which creep rupturestrength over 100 N/mm² is reliably obtained, (Al %+0.1×Ni %) is limitedto 0.02% or less.

Copper (Cu) has an effect of restraining production of δ ferrite whenmixed as an impurity into copper as in the case of Co. However, mixtureof Cu may lower the long-term creep rupture strength at a temperature of600° C. or more, so that it is limited to 0.3% or less. Cu may not becontained.

Boron (B) is a grain boundary strengthening element (element thatstrengthens crystal grain boundaries), and remarkably increases thecreep rupture strength even at a small amount. It dissolves into M23C6carbide and restrains aggregation and coarsening of M23C6 carbide toincrease the creep rupture strength, so that at least 0.001% of B isadded. However, if 0.010% or more of B is added, weldability of thesteel is greatly deteriorated, so that the additive amount of B is setto less than 0.10%.

The major chemical component ranges of the ferritic heat-resistant steelof the present invention are as described above, however, it may containamounts of the following elements less than the contents described belowin units of percent by weight as impurities.

Ta<0.2%, Ti<0.1%, Zr<0.2%, La<0.1%, Ce<0.1%, Pd<0.2%, Re<0.5%, Hf<0.3%

These elements also have the effect of increasing the strength asfollows:Ta: forms TaC and strengthens the base metal.Ti: forms TiC and strengthens the base metal.Zr: forms ZrC and strengthens the base metal.La, Ce: lowers the proportion of oxygen in the steel and increases thecreep rupture strength.Pd: improves the creep rupture strength and oxidation resistance (steamoxidation resistance).Re: strengths the base metal.Hf: forms HfC and strengthens the base metal.

The ferritic heat-resistant steel of the present invention is normalizedat a temperature of 1,050 to 1,100° C. and tempered at a temperature of750 to 800° C. after molten and forged, and then used as a temperedmartensite tissue. From the standpoint of obtaining toughness, a singlephase of tempered martensite tissue is desirable. However, when it isused as a high-temperature boiler member, in the case where some degreeof lowering in toughness is permitted, it is allowed that the amounts offerrite forming elements such as Cr and Si, etc., are set to becomparatively large in the limited ranges described above to precipitateδ ferrite. In view of toughness and creep rupture strength, it is knownthat a volume ratio of δ ferrite over 35% lowers the strength andtoughness, so that the volume ratio is limited to 30% or less.

The steel of the present invention is characterized by containing Creduced to the half, lowered Al and Ni, and increased Si in comparisonto the idea of the conventional high-Cr heat-resistant steel. By thecompound effect of these, the stability of the creep strength isimproved for the first time, and at the same time, the oxidationresistance (steam oxidation resistance) is improved, whereby a high-Crferritic heat-resistant steel usable up to 650° C. is achieved. It ispossible to employ various manufacturing methods according to thepurpose of use of the steel, and the steel can be used not only as asteel pipe but also as a steel sheet.

In addition, the ferritic heat-resistant steel of the present inventionhas creep rupture strength remarkably improved in comparison to theconventional ferritic heat-resistant steel, and has strength andductibility stable even during long-term use. Therefore, by applyingthis steel to a high-temperature pressure resistant portion of an ultrasupercritical pressure boiler, the steam temperature can be raised toabout 650° C., and the plant efficiency of a thermal power plant can beimproved. Further, growth and exfoliation of steam oxidized scale anddamage of devices due to scattering of steam oxidized scale can bereduced. Therefore, the durability of the plant is also improved, and aremarkable effect in fuel consumption reduction such as coal and CO₂emissions reduction in the thermal power plant can be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing results of the oxidation test using steamwhen the Si amount was changed in 9CrWCo-based steel of the presentinvention;

FIG. 2 is a diagram showing results of the creep rupture test when the Wamount was changed in the 9CrWCo-based steel of the present invention;

FIG. 3 is a diagram showing results of the creep rupture test by showingthe Al amount on the horizontal axis when the Al and Ni amounts werechanged in the 9CrWCo-based steel of the present invention; and

FIG. 4 is a diagram of the same test results as of FIG. 3 by rewritingthe Al amount on the horizontal axis into (Al+0.1×Ni) amount.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, examples/embodiments of the present invention will bedescribed by using actual examples.

The heat-resistant steels having the chemical compositions shown inTable 1 of the present examples and comparative steels were molten in avacuum induction melting furnace, and forged into 50 kg ingots,respectively. The comparative steel A is nominal 9Cr1MoNbV steel,comparative steels B and C are nominal 9Cr0.5Mo1.8WNbV steels, and allof these have been made practicable as boiler steels. After formingsteel sheets with a thickness of 20 millimeters by hot forging, thesteel sheets were normalized at 1,050° C. for 60 minutes and tempered at780° C. for 60 minutes, and then subjected to a creep rupture test. Inaddition, small-sized sheet-like test specimens were processed from thesteel sheets, and subjected to an oxidation test by using steam at 650°C.

TABLE 1 (wt %) C Si Mn P S Cr Mo W V Nb Ni Steel A of the presentinvention 0.043 0.51 0.50 0.003 0.001 8.9 0.45 1.80 0.19 0.051 0.005Steel B of the present invention 0.048 0.43 0.49 0.002 0.001 9.0 0.471.71 0.20 0.057 0.008 Steel C of the present invention 0.075 0.40 0.480.003 0.001 9.2 0.50 1.90 0.18 0.060 0.007 Steel D of the presentinvention 0.019 0.60 0.52 0.003 0.001 9.5 0.45 1.81 0.21 0.059 0.010Steel E of the present invension 0.012 0.75 0.51 0.002 0.001 8.8 0.402.00 0.18 0.050 0.015 Steel F of the present invention 0.035 0.55 0.500.002 0.001 9.0 0.30 2.02 0.22 0.055 0.010 Comparative steel A 0.1000.40 0.40 0.008 0.004 8.9 1.02 — 0.21 0.058 0.28 Comparative steel B0.110 0.30 0.35 0.010 0.006 9.0 0.48 1.81 0.20 0.062 0.22 Comparativesteel C 0.082 0.07 0.42 0.008 0.005 9.1 0.52 1.76 0.19 0.053 0.20 Co N BAl Cu Mo + 0.5W C + N 0.1Ni + Al Steel A of the present invention 1.810.046 0.006 0.0010 0.005 1.350 0.089 0.0015 Steel B of the presentinvention 1.91 0.043 0.005 0.0008 0.005 1.325 0.091 0.0016 Steel C ofthe present invension 1.70 0.05 0.007 0.0012 0.005 1.450 0.125 0.0019Steel D of the present invention 1.75 0.062 0.006 0.0009 0.004 1.3550.081 0.0019 Steel E of the present invention 1.82 0.070 0.005 0.00100.005 1.400 0.082 0.0025 Steel F of the present invention 1.75 0.0550.006 0.0009 0.004 1.310 0.090 0.0019 Comparative steel A 0.00 0.0480.000 0.007 0.03 1.02 0.15 0.0350 Comparative steel B 0.00 0.051 0.0060.005 0.05 1.39 0.16 0.0270 Comparative steel C 0.00 0.043 0.006 0.0060.04 1.40 0.13 0.0260

100,000-hour creep rupture strengths at 600° C. and 650° C. estimated byplotting the results of the creep rupture test of the steels of theexamples of the present invention and the comparative steels bytemperature as a stress-rupture time diagram and extrapolating these onthe long-term side were shown in Table 2.

TABLE 2 10⁵ h creep rupture strength (N/mm²) 600° C. 650° C. Steel A ofthe present invention 182 105 Steel B of the present invention 185 103Steel C of the present invention 188 107 Steel D of the presentinvention 185 105 Steel E of the present invention 180 102 Steel F ofthe present invention 185 106 Comparative steel A 92 48 Comparativesteel B 135 68 Comparative steel C 140 70

The steels A and B of the present invention have creep rupture strengthsat 650° C. after 100,000 hours which are about twice the creep rupturestrength of the comparative steel A that was used as a conventionalboiler heat-resistant steel for many years, and are about 1.5 times thecreep rupture strength of high-strength comparative steels B and C, sothat A and B have revolutionary strengths. The results of the oxidationtest using steam conducted for the steels of the examples andcomparative steels are shown in Table 3, and the results show that thegrowth of oxidized scale due to steam is restrained with respect to thecomparative steels, so that it is considered possible that the steels ofthe present invention are sufficiently usable at a steam temperature of650° C.

TABLE 3 Water vapor oxidized scale thickness (μm) (650° C., 2000 h)Steel A of the present invention 120 Steel B of the present invention150 Steel C of the present invention 150 Steel D of the presentinvention 115 Steel E of the present invention 80 Steel F of the presentinvention 120 Comparative steel A 160 Comparative steel B 200Comparative steel C 300

In the present invention, the experiments were performed by setting thenormalizing temperature to 1,050° C., however, a higher creep rupturestrength can be obtained by raising the normalizing temperature.However, at the same time, the toughness lowers, so that the normalizingtemperature is preferably in the temperature range up to 1,100° C.

The ferritic heat-resistant steel of the present invention is suitable,in particular, as a material of a header and a main steam pipe of asuperheater of an ultra supercritical pressure boiler with a steamtemperature around 650° C. The ferritic heat-resistant steel of thepresent invention can be used not only as a thick and large-diameterpipe material but also as a small-diameter heat transfer pipe material.

INDUSTRIAL APPLICABILITY

The ferritic heat-resistant steel of the present invention is highlyindustrially applicable not only as a material of a header and a mainsteam pipe of a superheater and a thick and large-diameter pipe materialof an ultra supercritical pressure boiler with a steam temperaturearound 650° C. but also as a small-diameter heat transfer pipe material.

1. A ferritic heat-resistant steel which has the following chemicalcomposition (by weight): carbon (C): from 0.01% to less than 0.08%;silicon (Si): 0.30-1.0%; phosphorus (P): 0.020% or less; sulfur (S):0.010% or less; manganese (Mn): 0.2-1.2%; nickel (Ni): 0.3% or less;chromium (Cr): 8.0-11.0%, molybdenum (Mo): 0.1-1.2%; tungsten (W):1.0-2.5%; vanadium (V): 0.10-0.30%; niobium (Nb): 0.02-0.12%; cobalt(Co): 0.01-4.0%; nitrogen (N): 0.01-0.08%; boron (B): not less than0.001% and not more than 0.010%; copper (Cu): 0.3% or less; aluminum(Al): 0.010% or less; provided that the amount of (Mo %+0.5W %) is1.0-1.6, and the amount of (C %+N %) is 0.02-0.15%, and which comprisesa tempered martensite single-phase tissue produced by thermal refiningand comprises 30% by weight or less of 5 ferrite.
 2. The ferriticheat-resistant steel according to claim 1, having the chemicalcomposition in which the amount of (Al %+0.1Ni %) is 0.02% by weight orless.
 3. The ferritic heat-resistant steel according to claim 1,comprising from 0.01% to 0.075% carbon.
 4. The ferritic heat-resistantsteel according to claim 2, comprising from 0.01% to 0.075% carbon. 5.The ferritic heat-resistant steel according to claim 1, comprising0.40-0.75% by weight of Si.
 6. The ferritic heat-resistant steelaccording to claim 2, comprising 0.40-0.75% by weight of Si.
 7. Theferritic heat-resistant steel according to claim 3, comprising0.40-0.75% by weight of Si.
 8. The ferritic heat-resistant steelaccording to claim 4, comprising 0.40-0.75% by weight of Si.
 9. Theferritic heat-resistant steel according to claim 1, further comprising(by weight) at least one of <0.2% Ta, <0.1% Ti, <0.2% Zr, <0.1% La,<0.1% Ce, <0.2% Pd, <0.5% Re, <0.3% Hf.
 10. The ferritic heat-resistantsteel according to claim 2, further comprising (by weight) at least oneof <0.2% Ta, <0.1% Ti, <0.2% Zr, <0.1% La, <0.1% Ce, <0.2% Pd, <0.5% Re,<0.3% Hf.
 11. The ferritic heat-resistant steel according to claim 3,further comprising (by weight) at least one of <0.2% Ta, <0.1% Ti, <0.2%Zr, <0.1% La, <0.1% Ce, <0.2% Pd, <0.5% Re, <0.3% Hf.
 12. The ferriticheat-resistant steel according to claim 4, further comprising (byweight) at least one of <0.2% Ta, <0.1% Ti, <0.2% Zr, <0.1% La, <0.1%Ce, <0.2% Pd, <0.5% Re, <0.3% Hf.
 13. The ferritic heat-resistant steelaccording to claim 5, further comprising (by weight) at least one of<0.2% Ta, <0.1% Ti, <0.2% Zr, <0.1% La, <0.1% Ce, <0.2% Pd, <0.5% Re,<0.3% Hf.
 14. The ferritic heat-resistant steel according to claim 6,further comprising (by weight) at least one of <0.2% Ta, <0.1% Ti, <0.2%Zr, <0.1% La, <0.1% Ce, <0.2% Pd, <0.5% Re, <0.3% Hf.
 15. The ferriticheat-resistant steel according to claim 7, further comprising (byweight) at least one of <0.2% Ta, <0.1% Ti, <0.2% Zr, <0.1% La, <0.1%Ce, <0.2% Pd, <0.5% Re, <0.3% Hf.
 16. The ferritic heat-resistant steelaccording to claim 8, further comprising (by weight) at least one of<0.2% Ta, <0.1% Ti, <0.2% Zr, <0.1% La, <0.1% Ce, <0.2% Pd, <0.5% Re,<0.3% Hf.